The CTO of a Costa Rica-based payment network that U.S. prosecutors allege primarily served the cybercriminal underworld pleaded guilty on Thursday to one count of conspiring to operate an unlicensed money transmitting business.

Mark Marmilev, 35, of Brooklyn, could face a maximum of five years in prison, according to a news release from the U.S. Attorney for the Southern District of New York.

Marmilev was accused of designing and maintaining the systems of Liberty Reserve, a digital currency used by a million people worldwide until it was shut down in May 2013. Prosecutors allege that Marmilev knew Liberty Reserve was transmitting funds that came from criminal activity.

Liberty Reserve transmitted US$6 billion worth of transactions from its launch in 2006, mostly with the intent to launder criminal funds from activities such as credit card fraud, identity theft, hacking and narcotics trafficking, the indictment said.

The company functioned “as a bank of choice for the criminal underworld because it provided an infrastructure that enabled cybercriminals around the world to conduct anonymous and untraceable financial transactions,” prosecutors said.

Marmilev was one of seven people indicted. Two others, Vladimir Kats and Azzeddine el Amine, have pleaded guilty to charges and are awaiting sentencing.

Liberty Reserve did not require its customers to present identification, allowing them to stay anonymous or use fake names. Third-party exchangers bought and sold credits from Liberty Reserve in exchange for mainstream currency.

Those exchangers, mostly in Malaysia, Russia, Nigeria and Vietnam, dealt directly with customers, according to the indictment. That arrangement ensured that Liberty Reserve never collected identifying information on its users.

In 2009, Costa Rican authorities pressed Liberty Reserve for a license to operate as a money transmitter. It failed to obtain the license after regulators in the country determined it did not use anti-money laundering controls and had no means to track suspicious activity.

It was finally shut down in May 2013 after Liberty Reserve allegedly engaged in various deceptions, including moving money outside of Costa Rica into the accounts of shell companies and claiming it was shutting down its business.

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